1. Field of the Invention
The present invention relates to a mobile IP network system and a connection switching method. More particularly, the invention relates to a technique of switching a logical connection for IP packet communications between a radio access network and a packet node connected to an IP (Internet Protocol) network in association with movement of a mobile station.
2. Description of the Related Art
In order to enable IP data to be transferred between a mobile station and a host connected to an IP network typified by the Internet, a network system in which a mobile communication network is connected to the IP network via a packet node having a foreign agent function in a mobile IP is being examined.
The mobile IP denotes a technique for enabling the communication between a mobile station and a host to be continued without changing an IP address even when the connection position between the mobile station and the IP network changes. In the mobile IP, a node having a home agent function for each of mobile stations is preliminarily determined. The home agent function denotes a function of grasping the current position of mobile stations under the control, when an IP packet destined for a mobile station under the control is received, converting the reception packet to an encapsulated packet destined for a packet node connected to a radio access network in which the mobile station is positioned at present, and transmitting the encapsulated packet to the IP network.
Each of packet nodes for receiving the encapsulated packet via the IP network has a foreign agent function. The foreign agent function denotes here a function of decapsulating the reception packet and transmitting the obtained IP packet to a radio access network in which a destination mobile station is positioned.
By the home agent function and the foreign agent function, even when a mobile station moves between radio access networks, an IP packet can be delivered to the mobile station without changing the IP address of a transmission packet.
In the following, a node having the home agent function will be called a home agent node and a node having the foreign agent function will be called a foreign agent node.
A communication between a mobile station and a host connected to an IP network is realized by switching a logical connection between the mobile station and a foreign agent node by using a position control function peculiar to a mobile communication network and routing an IP packet between the foreign agent node and the host by the mobile IP function.
The mobile IP is standardized by IETF and is described in, for example, “IP Mobility Support”, C. Perkins, RFC2002, October 1996. A mobile IP data communication in IMT-2000 as a third-generation mobile communication network is described in “Wireless IP Network Architecture based on IETF Protocols”, Tom Hiller, 1999 disclosed by TIA as a standardization organization in North America.
In the mobile IP, when a mobile station is moved from a control area of a packet node (foreign agent node) to a control area of another packet node, a registration procedure for switching the foreign agent node is necessary in the home agent node.
The mobile station notifies the home agent node of a destination packet node and the home agent node registers the destination packet node as a new foreign agent for the mobile station. Consequently, the home agent node can transfer an IP packet destined for the mobile station received after that to the new foreign agent (packet node on the destination side).
The conventional technique has a problem such that it takes time from the request of the registration procedure to the response. Specifically, when a mobile station moves to a control area of another packet node and is handed over to a radio base station in the control area, until the registration is completed in the home agent node, IP packets destined for the mobile station are transferred from the home agent node to a packet node in the previous control area, so that the IP packets are not transferred to the mobile station. Although the dropped IP packets may be compensated by retransmission of the IP packets from the host in response to a request from the mobile station, when the amount of the dropped IP packets is large, a problem such that the retransmission of the IP packets severely deteriorates the throughput occurs.